The popularity of plasma processing continues to grow throughout the semiconductor industry. Although some new equipment is being designed to operate in microwave frequencies, the vast majority of both new and installed tools operate in the radio frequency or RF band of between 0.1 to 13.56 MHz. Although various process tools explore the use of frequency attributes, for the most part RF power is only important to plasma generation. This fact is important, because the repeatability of the plasma process almost entirely depends on the repeatability of the plasma.
As the principal driving force in plasma formation, the RF frequency and power should be the most carefully controlled and best understood machine parameter. Unfortunately, this is most typically not the case. FIG. 1, for example, shows RF network 10 that produces RF power for an electronic device and fabrication reactor. In particular, for producing RF plasma generation, RF power generator 12 connects to local automated matching network 14 via cable 16. From local automated matching network 14, mechanical RF connection 18 goes to process chamber 20. Process chamber 20 includes cathode 22 that affects process gas 24 within plasma sheath 26 to produce an RF plasma.
In RF network 10, certain limitations exist. For example, RF power generator 12, while including solid state technology, still is a bulky system that consumes an undesirable amount of clean room floor space. As a result, performance of RF network 10 is often adversely affected by installation dependencies due to generator placement. The objective of local automated matching network 14 is to provide an efficient transfer of RF power from the RF power generator 12 to the RF load of plasma process gas 24 by matching the widely different impedances between RF power generator 12 and process chamber 20 (the RF load). Unfortunately, this match is accomplished by non-ideal impedance elements. This is particularly unfortunate considering the fact that impedance monitoring is a fundamental criterion in the frequency bands of interest to semiconductor plasma processing.
A further limitation of RF network 10 relates to process chamber 20 itself. Within process chamber 20, the electronic device, such as a semiconductor wafer, is positioned and processed to achieve some desired result such as etch or deposition. With regard to process chamber 20, two significant limitations exist. First of all, even with known installation dependencies and variability due to the local automated matching network 14, the RF power is primarily controlled based on a measurement made at RF power generator 12. Furthermore, even though RF power generator 12 for a given power level consists of three variables of voltage, current and phase angle, known systems generally measure and control RF power with the unit of watts only.
FIGS. 2 and 3 illustrate the limitations that exist when RF load power is measured only in terms of watts. In FIG. 2 appears diagram 30 that includes origin 32 having axes 34, 36 and 38. Axis 38 denotes the power level for RF power from RF power generator 12. In particular, three-dimensional object 40 depicts the variability that exists with a one-dimensional measure of power. Consider, for example, that for a given power level, the intersection of plane 42 within surface 40 depicts a minimum reflected power. Suppose further that axis 34, for example, represents changes in voltage for a given power level, while axis 36 represents variation in current at the same power level. As FIG. 2 illustrates, for a given power level and minimum reflected load power considerable variability in current and voltage is possible. FIG. 2, however, does not even consider phase angle differences.
FIG. 3 shows an actual measurement example of changes that can occur with a given power level in the three variables of voltage, current and phase angle. In particular, three-dimensional plot 50 shows on axis 52 changes in phase angle between -81 and -87 degrees. On axis 54 appears a scale of RMS current, A.sub.RMS ranging from 1.40 to 1.56 A.sub.RMS. Along axis 56 are voltage measurements between 522 and 528 V.sub.RMS. As measurement point 58 indicates, for a given minimum RF power to process chamber 20, measurement points 58 exhibit a significant amount of variability. This obviously can significantly affect RF plasma measurement within process chamber 20.
Local automated matching network 14 of RF network 10 may be one of a wide variety of designs, all of which operate by tuning to a minimum reflected power. The dead band concept of matching circuit tuning, where inductive and/or capacitive elements are varied which drive the circuit to an acceptable reflected power level is essentially the concept that FIG. 2 depicts. As FIGS. 2 and 3 illustrate, this concept of only monitoring reflected power could produce an infinite number of voltage, current, and phase angle combinations. For these infinite combinations associated differences in plasma and process parameters relate. The real world example that FIG. 3 illustrates is for a single requested forward/reflected power combination. Thus, data points 58 show the potentially infinite combinations that a widely accepted plasma tool generates operating under nominal conditions.
An additional problem with known RF power networks for producing plasma environments in process chambers, such as process chamber 20, is how known systems treat the RF load from RF generator 14. Known systems treat the RF load as the discharge between two electrodes in process chamber 20. Electronic device fabrication process techniques, however, also treat as important the wall chemistry within process chamber 20. This produces at least one more RF load for RF power from RF generator 12 that the known RF networks ignore. As a result, RF power measurements of the radio frequency power that goes to process chamber 20 to produce process plasma 24 generally do not precisely measure the RF power that the electronic device receives.